FIG. 1 shows a conventional electric field sensor. The electric field sensor comprises a sensor head 1 and an antenna 2 connected to the sensor head 1. The sensor head 1 has a substrate 3 and an optical modulator 4 attached to the substrate 3.
The optical modulator 4 comprises an incident optical waveguide 5 formed on the substrate 3, two phase-shift optical waveguides 6 which are formed on the substrate 3 to be branched from the incident optical waveguide 5 and each of which has a variable refractive index varying in response to an electric field intensity applied thereto, an outgoing optical waveguide 7 formed on the substrate 3 to join the phase-shift optical waveguides 6, and two modulation electrodes 8 formed on or in the vicinity f the phase-shift optical waveguides 6. The incident optical waveguide 5 is connected to an incident optical fiber 9. The outgoing optical waveguide 7 is connected to an outgoing optical fiber 10.
The antenna 2 has two rod antenna elements 11. The rod antenna elements 11 are connected through lead wires 12 to the modulation electrodes 8, respectively. The rod antenna elements 11 are arranged to face a direction of an electric field, in other words, to be perpendicular to the phase-shift optical waveguides 6. The rod antenna elements 11 are fixed to a package which is not illustrated in the figure.
In the conventional electric field sensor, the antenna elements face the direction of the electric field during measurement. Accordingly, a draw-out portion of the optical fiber faces a direction of measurement. This possibly results in a damage of the optical fiber.
In the conventional electric field sensor, the antenna must be arranged in various directions during measurement because of its strict directivity.
It is an object of this invention to provide an electric field sensor capable of preventing a damage of an optical fiber.
It is another object of this invention to provide an electric field sensor which need not be arranged in various directions during measurement.